The need for smaller and faster computer chips has caused a dramatic increase in the power needed to remove from the chip. This is made more difficult by the shrinking of the die and the larger heat flux per unit area. Thermal interface materials (TIMs) have a key function in a flip chip package, i.e. to dissipate heat to allow higher processing speeds. More specifically, thermal interface materials bring the die into good thermal contact with the heat removal hardware.
Thermal interface materials are available in a wide variety of formulas from silicone and non-silicone bases filled with metal oxides. The metal oxide particles provide the high thermal conductivity to the compound. The ability to fill the tiny cavities of mating surfaces will depend on the metal oxide particle sizes. The particles are designed to give the highest thermal conductivity to the compound. The lowest thermal resistance is a combination of high thermal conductivity and the ability of the material to penetrate all of the cavities and fill all the spaces created by any non-flat areas of the two mating surfaces. Thermal grease provides the lowest thermal resistance interface available (not including a soldered type connection). The disadvantage of thermal grease is the inconsistency of application and the problem of keeping it from being messy to use. There are many grease application products available today to help with the ease of use and keeping it where it belongs, such as spraying, screening, sticks and pads (pads are a grease that is dry to the touch).
Attaching a heat sink to a semiconductor package requires that two solid surfaces be brought together into intimate contact. Unfortunately, no matter how well prepared, solid surfaces are never really flat or smooth enough to permit intimate contact. All surfaces have a certain roughness due to microscopic hills and valleys. Superimposed on this surface roughness is a macroscopic non-planarity in the form of a concave, convex or twisted shape. As two such surfaces are brought together, only the hills of the surfaces come into physical contact. The valleys are separated and form air-filled gaps. When two typical electronic component surfaces are brought together, less than one percent of the surfaces may make physical contact with the remainder (99%) of the surfaces separated by a layer of interstitial air. Some heat is conducted through the physical contact points, but much more has to transfer through the air gaps. Since air is a poor conductor of heat, it should be replaced by a more conductive material to increase the joint conductivity and thus improve heat flow across the thermal interface.
Several types of thermally conductive materials can be used as TIMs to eliminate air gaps from a thermal interface including greases, reactive compounds, elastomers, and pressure sensitive adhesive films. All are designed to conform to surface irregularities, thereby eliminating air voids and improving heat flow through the thermal interface.
A TIM can be made from a polymer matrix and a highly thermally conductive filler. TIMs find three application areas in a CPU package: 1) to bring a bare die package into contact with a heat spreader (FIG. 1A), 2) to bring the die into good thermal contact with an integrated heat sink hardware (FIG. 1B), and 3) to bring the heat spreader into contact with OEM applied hardware (FIG. 1B). The TIM placed between the die or die package and heat spreader is called a TIM 1 and the TIM placed between the heat spreader and heat sink hardware is referred to as a TIM 2.
Historically, soft polymers used in TIMs have been silicones, epoxies, urethanes, acrylates and olefins. Filler types have ranged dramatically from inexpensive aluminum oxides and zinc oxide to aluminum, boron nitride, silver, graphite, carbon fibers, and diamond. Phase change TIMs are a class of polymer materials that undergo a transition from a solid to a liquid phase with the application of heat. The phase change TIMs are a soft solid at room temperature but a thick fluid at operating temperature. This transition occurs due to the presence of a low melting solid, typically a wax, mixed with the polymer in the presence of highly conductive filler. Due to the transition, phase change materials readily conform to surfaces and provide low thermal resistance and higher heat removal capability.
A heatpipe is a heat transfer or heat sink structure that can include a number of channels for transferring heat from one end to a condenser region at the other end. Each heatpipe can be composed of a central vapor channel with a number of parallel capillary channels (not shown), each of which is open on one side to the vapor channel thereby serving as the wick of the heat pipe, running the length of the heatpipe to a condenser region. The heat from the microchip vaporizes a working fluid in the capillaries and the vapor in turn travels in the vapor channel to the condenser region to be cooled and condensed by a cooling medium, such as air, present over this region.
When a heatpipe is used, a heatpipe surface contacting the circuit package can have a cross-section smaller than the circuit package it contacts and a portion of the circuit package may extend out beyond the heatpipe edges. As a result, heat transfer may not be as efficient as required and a thermal adaptor such as a spreader plate may be used as a heat spreader to compensate. To improve thermal conduction between the heatpipe and the circuit package, the spreader plate can have a surface area and shape that more closely matches with the circuit package when the spreader plate is positioned between the heatpipe and the circuit package.
FIG. 2 is an illustration of an arrangement of a non-fusible particle filler material within the polymer matrix of a TIM. The non-fusible particles, such as metals, benefit from a high thermal conductivity, however, a thermal flow path through the TIM is limited by the point-to-point contact of the particles as shown by the arrows. Within the TIM, these particles being non-fusible (i.e. will not melt and flow during normal processing and so remain as point contacts with each other) result in thermal conductivity through the TIM that is a mechanism referred to as percolation. The phenomenon of percolation describes the effects of interconnections present in a random system, here the number of filler particles that are randomly in point contact with each other to allow thermal conduction. Normally, to improve conduction limited by percolation, the amount of filler could be increased until a threshold amount is reached and heat conduction due to the filler, transitions to a sufficiently high value. The degree of filler required to reach this transition level may be too high and can overpower the properties desired from the polymer binder such as low contact resistance. Another problem is that for some metal particles in contact with some polymer binders, the bare particle filler can poison the polymer cure such as by hindering or blocking the curing agent.